Field of the Invention
Embodiments of the invention generally relate to piston compressors for compressing gas, and more particularly to a horizontal piston compressor incorporating a free floating piston arrangement.
Discussion of Related Art
Horizontal piston compressors are generally known. Such piston compressors of are generally very large double-acting compressors with several cylinders and are used in the oil and petrochemicals industry. The forces of inertia which are the result of the large mass of the reciprocating parts of the compressor are a major reason for placing the cylinders horizontally in the frame. Although a large part of these forces can be compensated for by balancing the movements of the piston/piston rod units, the remaining forces on the frame of the compressor can be absorbed more readily by the bedplate of the compressor if they are directed horizontally instead of vertically.
Horizontal piston compressors suffer from a generally known problem with regard to supporting the reciprocating piston/piston rod unit relative to the stationary part of the compressor (i.e. the frame and the cylinder(s) forming part thereof). In general, a piston/piston rod unit is supported at the crosshead side by the crosshead which is guided in the frame, and at the other side the piston rests on the bottom part of the wall of the cylinder. The piston is often provided with one or more replaceable belts, which lie around the piston in the peripheral direction and project beyond the body of the piston. These belts are known as rider rings.
Over time, wear of the rider rings leads to run-out, which is permissible only within certain limits. Oil has generally been used as the lubrication between the piston and the cylinder wall in order to prevent excessive wear of the bearing surfaces and minimize the occurrence of run-out. The problem with oil lubrication, however, is that the lubricating oil can contaminate the compressed gas. As such, there is a continuing need for “oil free” compressors. To make an “oil free” compressor requires careful selection of the material of the rider rings and their fastening to the piston. In some cases the rider rings are made from materials with advantageous lubricating and wear properties, such as polytetrafluoroethylene (PTFE), commonly known as Teflon.
As previously noted, horizontal piston compressors are often used in situations where continuous operation is required. And although the mechanical construction of such compressors has developed so that the compressors can operate continuously at high efficiency for years, the wear rate of the rider rings is greater than desired. Thus, in practice the compressors have to be shut down after a few months in order to measure the wear on the rider rings and in order to be able to replace any rings which may be worn to unacceptable levels.
This maintenance adversely affects the overall efficiency and serviceability of this type of compressor. It would, therefore, be desirable to provide an improved bearing arrangement between the piston and the cylinder of the compressor which makes it possible to operate a compressor continuously for considerably longer periods than current compressors.